Macroevolution






  • Microevolution happens at the population level (natural selection, genetic drift, gene flow)


  • Macroevolution: Speciation events form evolutionary independent populations


  • New species are the bridge between evolution within a population and larger patterns in evolution, such as new taxonomic groups

How/Why species form: The limits of human cognition


What actually is a “Species”?








  • Textbooks: A group of populations whose members have the potential to interbreed in nature and produce viable offspring

What actually is a “Species”?








  • Textbooks: A group of populations whose members have the potential to interbreed in nature and produce viable offspring

Biological Species Concept: Unity of reproduction



  • Reproductive isolation between populations results in a lack of gene flow
    • what are the consequences of no gene flow?


  • Individuals do not interbred with other populations or are unable to produce viable offspring after mating
    • pre- and post-zygotic


  • BSC: Members of a biological species are united by the potential ability to be reproductively compatible

Mechanisms of reproductive isolation? (Fig 24.3)



Issues with the biological species concept




  • How do we effectively evaluate isolation?
    • Is this even possible in fossils?


  • Geographic proximity: how do we know if they cant make babies?


  • Asexual reproduction
    • Belloid rotifers haven’t reproduced sexually for > 80 million years
    • An estimated 2000 species of asexual rotifers are known

Morphological Species Concept: Unity of body plans


Issues with the morphological species concept





  • Applies to all organisms but…. what kinds of traits & how many?
    • Subjective to the scientist!


  • Species can change their form: Polymorphisms


  • Cryptic species that do not differ in morphology
    • lots of mimics in nature!

Phylogenetic Species Concept: Unity of ancestry


How new species arise: Allopatry


Allopatric speciation in Hawaiian fruit flies


Allopatric speciation in shrimp


How new species arise: Sympatry


Sympatry in Hawthorn and apple flies


How much genetic change does it take?



  • Three spine stickleback fish rapidly evolve
    • moved from oceans to freshwater when glaciers retreated after last ice age


  • Freshwater habitats brought new selection pressures
    • variety of habitats
    • new resources
    • change in competition


  • Molecular biology examined each speciation
    • ocean → fresh: one gene with bony armor plate
    • fresh habitats → many genes based on selection pressures

Study aid: The geography of speciation


The pre- and postzygotic barriers can form for either type of speciation